Abstract : The ease with which pentachlorobenzene sulfenyl chloride (I) undergoes a series of photochemicallyinduced reactions is demonstrated. In ordinary, diffuse sunlight, but not in the dark, (I) undergoes complete reaction with excess of cyclohexane, yielding hydrogen chloride, cyclohexyl chloride, cyclohexyl pentachlorophenyl sulphide, m.p. 116C and bis(pentachlorophenyl) disulphide, m.p. 235-236C. A chain-radical process is proposed for this reaction. Compound (I) also undergoes a light-catalysed reaction with toluene, only the side chain being affected in the absence of Friedel-Crafts catalysts. Other light-induced reactions of (I) which illustrate its high reactivity, are its facile additions to diphenylacetylene and to deactivated olefins, and its light-induced decomposition in carbon tetrachloride solution to form bis(pentachlorophenyl) disulphide. Another new reaction is that between (II) and pbenzoquinone. This reaction also requires illunination and leads to several products, including quinhydrone, 2-pentachlorophenylsulphenyl-1:4-vebenzoquinone m.p. 177C), and substitution products thereof, including a disulphenylated benzoquinone, decomp. >300C, and a chloro-derivative, m.p. 219-220C. (Extracted)